1. Field of the Invention
This invention relates generally to flexible fans which automatically reduce pitch in response to increased rotational speed.
2. Description of the Prior Art
In water-cooled internal combustion engine-powered automotive vehicles, the engine-driven fan must draw a large volume of cooling air through the radiator when the vehicle is stationary or operating at low speed whereas, at high vehicle speeds, the movement of the vehicle itself produces much of the required cooling airflow through the radiator and thus, the demand for air movement by the fan is significantly decreased. In fact, in certan larger air conditioned automobiles, a fan clutch is provided to disable the fan at high engine speeds in order to prevent excessive cooling.
U.S. Pat. No. 3,758,231 to the present applicant and assigned to the assignee of the present application describes and illustrates a molded plastic flexible fan which automatically reduces its pitch in response to increased rotational speed. That fan includes a plurality of blades radiating from a hub, each blade having a root portion joined to the hub, a tip portion, leading and trailing edges, a relatively stiff leading portion and a relatively flexible trailing portion joined to the leading portion by a relatively narrow intermediate or hinge zone which extends diagonally across the blade from generally the trailing edge of the root portion to generally the leading edge of the tip portion.
The felxible fan shown and described in the aforesaid patent was never produced as a one-piece, molded unit, the cost of a mold being too high by reason of the overlap between the leading and trailing edges of certain adjacent blades at the root thereof which required a side-pull on the mold.
In an effort to solve the mold cost problem, I provided a notch in the trailing edge of each blade at the root so as to eliminate the overlap of certain blades. However, when that fan was molded as a one-piece unit, it was found that a failure mode was encountered in high speed cycle tests, i.e., repeated cycling from zero to a high speed such as 6750 rpm. Such failure is in the nature of a fracture which starts at the trailing edge of a blade in the hinge zone and progresses outwardly in the hinge zone until slightly past the mid point therealong, at which point the fracture turns toward the trailing edge in some cases and toward the leading edge tip point in others.